Important criteria for choosing a good TE detector
When choosing a detector we would advise you to look at the sampling rate and (if possible) the quality and positioning of the microphone in / on the detector. The Pettersson D240X, for example, has a sampling frequency of 308 kHz, which allows one to record up to maximally 154 kHz (half the sampling rate). This is a bit low if you want to capture Myotis nattereri and emarginatus entirely. If you go abroad you won't be able to record Kerivoulas very well either. It is not just the upper frequency that is important, but also the general precision of the recording that is boosted significantly when using high sampling frequencies. It is as with digital cameras: you will see a clear improvement in resolution going from 6 Mpixels to 12 Mpixels, but beyond that your lens will be merely the limiting factor and not the camera resolution. It's pretty much the same with time expansion detectors. The difference in recording quality between 300 kHz and 600 kHz will be enormous. Sampling at 1 MHz will of course be even better, but you can be sure that already at 600 kHz the microphone will be the main limiting factor and not the sampling frequency.
Digital recording systems are also characterised by their bit depth, for example 8 bits in the D240X, or 16 bits in the Pettersson D500X. 8 bits mean 2^8=256 levels of encoding the amplitude and 16 bits mean 2^16=65536 levels. 8 bits are sufficient if the recording you made is as loud as it can be without being distorted (clipped). If the recording is quite weak because the bat was far away, the sound will be encoded in fewer than 256 levels, which is not very precise. Here, a 16 bit detector will still do a good job.
A very important factor is of course microphone quality. This is like the lens on a camera. When it comes to microphones, people are often quite fussed about the spectrum being flat. In my opinion, it's not a real problem if the sensitivity varies, or even diminishes across frequency, as long as there are no strange peaks or holes in the spectrum. With software it is very easy to correct the effects of a non-flat microphone as long as they are not too extreme. Still, after a recording has been corrected, the amplitudes at all frequencies will still be quite different from what the bat emitted, due to atmospheric attenuation. This will be much harder or even impossible to correct, so to the average bat worker a non perfectly flat microphone sensitivity really is not such a big deal.
What is really important is the overall sensitivity of the microphone, so the recordings are not very noisy. When it comes to this, Lars Pettersson knows what he's doing: both the D240 detectors with the small microphones and the D980 / D1000 are extremely sensitive and produce very little noise. For months, technicians at Tübingen University tried to develop the perfect condensator microphone, using different foils and condensators, but the best they ever achieved was just as good as the D980 microphone. Of course, it is also quite important to have sensitivity up to high frequencies, say up to 150-180 kHz. This should be specified by the manufacturer.
Another important factor that is sometimes overlooked by manufacturers is the placement of the microphone on or inside the detector. Possible problems with detectors having protruding condensator microphones are discussed under this link. This is the problem with ultrasound, it reflects everywhere, which is why bats decided to use it during evolution. If the microphone is placed close to reflecting elements, recordings may be affected seriously. In deciding what detector to use you should keep on eye on this. The only detector that meticulously eliminates this problem is the Ecoobs Batcorder.
Other considerations depend on your aims, how weatherproof it should be, how robust, how long battery-life should be, how rich you are etc. Detectors with a large microphone membrane may be damaged quicker because of rain than detectors with a small microphone membrane, but the latter ones might be less sensitive and therefore noisier. I hope this overview will help in making your choice.
Last updated 17.4.2009